1. Field of the Invention
The present invention relates to a process for the production of high quality refrigerator oil from vacuum distillate of low grade naphthenic crude oil (Grade B).
2. Description of the Prior Art
Refrigerator oil has heretofore been produced from high grade naphthenic crude oil (Grade A) (National Petroleum Refiners Association, "Naphthenic Luboil Feedstock Availabilities", 1981 Fuels and Lubricants Meeting) having a very low impurities (wax, naphthenic acid, polycyclic aromatics, etc.) content. A method of production of such refrigerator oil involves distillating the crude oil to remove the light fraction and the vacuum residue contained therein and subjecting the resulting fraction to such treatments as solvent extraction, sulfuric acid washing, clay treatment etc. to remove the remaining impurities.
The high grade naphthenic crude oil (Grade A) which is suitable for the production of refrigerator oil is in short supply and will tend to gradually decrease until it is no longer available.
Under these circumstances, various methods have been proposed to produce refrigerator oil from paraffinic crude oil, including a method as described in Japanese Patent Publication No. 11940/1965 in which a base oil feed from a paraffinic crude oil is subjected to an extraction procedure to adjust the polycyclic aromatic compound content to less than 8% by weight and, thereafter, it is hydrogenated under a mild condition and then is subjected to a clay treatment, and a method as described in U.S. Pat. No. 3,702,817 and Japanese Patent Publication No. 10082/1978 in which, in the extraction of a paraffinic base oil, hydrogenation is performed under such severe conditions that the resulting aromatics in the extract are ring-opened. In accordance with these methods, however, sufficiently satisfactory results cannot be obtained. In order to lower the pour point, it is necessary to employ mild hydrogenation conditions so as not to cause hydrocracking. Hydrogenation under such mild conditions, however, gives rise to the problems that impurities are not removed, resulting in the formation of a large amount of sludge, and chemical stability against a freon is reduced. If, however, hydrogenation is performed under severe conditions, although the impurity content is reduced, wax is formed by hydrocracking. Because of the formation of this wax, the pour point and the cloud temperature for the ultimate refrigerator oil are not lowered. It also causes flock (floating solids such as wax) which precipitates when the refrigerator oil is mixed with freon.
Venezuela crude oil is produced in a relatively large amount. Although Venezuela crude oil is a naphthenic crude oil, it contains large amounts of impurities such as naphthenic acid, polyaromatics, nitrogen compounds, etc. Accordingly, it is called "Grade B" and a method of producing high quality refrigerator oil from such crude oils has not yet been developed, and only low quality base oils for use in process oil, etc. are now produced.
The term "high quality refrigerator oil" is used herein to mean a refrigerator oil having a good fluidity at low temperatures and a low cloud point, and it is required to be thermally stable at high temperatures, not to form sludge, and not to be corrosive. It is further required for the high quality refrigerator oil to have compatibility with liquid freon at low temperatures and not to cause precipitation of flock of wax or the like and corrosion of the metal surface.